In the field of material processing, materials are often heated to cause a particular change in material morphology, a particular reaction to occur, or to cause a phase change. For example, in the area of conductive patterning, formulations or inks containing silver flakes or powder are laid down on a substrate and then heated to cause the particles to fuse and form a conductive line. In such case, the formulation is required to be fluid and often is nonconductive in order to print the pattern while at the end of the processing it must be solid and highly conductive. The heat changes the morphology of the silver to give the desired results. For silver inks, the temperature that the ink and substrate must be heated to in order to cure the ink is a function of the sintering temperature of the silver. For silver, the melting temperature is approximately 960° C. and the sintering temperature is approximately 800° C. This high temperature limits the substrates to materials that are unaffected by the high temperature. Many of the lower cost or flexible substrates such as cellulose (paper), Polyethylene Terephthalate (PET), Polyester and many other plastics cannot withstand these temperatures. Similarly, other components on the substrate, such as organic semiconductors may also decompose at elevated temperatures.
One approach to addressing this limitation is to use higher temperature substrates such as polyimide films. While this does provide a moderately high temperature flexible substrate, it is not necessarily high enough to form highly conductive films. Furthermore, it is expensive and not suitable for low cost applications.
Another approach to solve this problem is to use high loading of silver flakes in a resin or polymer that contracts during curing. This forces the silver flakes together causing them to make electrical contact. This approach has been demonstrated by Dow Corning under the trade name PI-2000 Highly Conductive Silver Ink. While this product appears to work in some applications, it does have some limitations in that it cannot be inkjetted.
Another approach to solving this temperature limitation is to use nanometals that exhibit a reduction in sintering temperature because of their small size. This approach has shown improvements by reducing the processing temperature to approximately 300° C.—approximately 700° C. Generally, to take advantage of the depressed sintering temperature, the particles must be discrete. Most nanometal synthesis processes such as SOL-GEL require chemical surface functionalization of the particles to keep the particles discrete and to prevent them from spontaneously fusing. This chemical surface functionalization generally needs to be volatilized at an elevated temperature that may be higher than the sintering temperature of the silver. Even if the surface functionalization is designed to evaporate below the sintering temperature, the sintering temperature may still be too high to use some of the lower temperature substrates. As the industry tries to lower the processing temperature, it is often done at the expense of the conductivity of the pattern. While it may be possible to use lower processing temperatures, the result is usually a pattern with less than adequate conductivity.
While the above example has been described in the context of conductive inks, there are similar applications where the same problems exist. For example, in catalytic applications, the catalyst is usually bonded to a high temperature substrate. In order for the reaction to occur at an acceptable rate, the catalyst must be at elevated temperatures. These high temperature substrates are often expensive and it is desirable to replace them with lower temperature substrates. Nanomaterials have begun to be used in these applications, because of their high reactivity and lower reaction temperatures. However, they still must operate at temperatures typically above the lower cost substrate's operating temperatures.
Therefore, in the field there exists a need to process materials at lower temperatures to allow more economical substrates to be used. More specifically, there is a need in the conductive patterning market to produce high conductivity patterns on low temperature substrates.